Recently, a relay station (RS) has been introduced to extend a cell coverage and increase a throughput. The relay station (RS) supports a conventional IEEE 802.16e-based mobile station (MS), and performs a function of an active repeater. The active repeater includes some functions of a base station (BS), a network entry and mobility of a PMP (Point-to-MultiPoint)-based RS, a radio resource management (RRM) function, and a security function.
FIG. 1 is a conceptual diagram illustrating a communication system to which a relay station (RS) is applied. In FIG. 1, the relay station (RS) decodes a signal received from a RS's reception end, re-encodes the decoded signal, and transmits the encoded signal to a RS's transmission end, such that noise can be removed. Also, the relay station (RS) serves as a digital amplifier capable of acquiring a high throughput using a higher date-rate encoding process.
The above-mentioned relay station (RS) can be classified into a fixed RS, a nomadic RS, and a mobile RS.
The fixed RS is permanently fixed, such that it is used to increase a cell coverage and shadow area. The fixed RS can also be used as a simple repeater.
The nomadic RS may be temporarily installed when the number of users suddenly increases, or may freely move in a building.
The mobile RS may be mounted to a variety of public transportations such as a bus or subway.
In the relay station (RS), a frame structure is modified into another frame structure to support the RS on the basis of a frame structure of a conventional OFDMA physical layer. Firstly, the conventional OFDMA frame structure will hereinafter be described in detail.
FIG. 2 is a structural diagram illustrating a frame structure of the conventional OFDMA physical layer.
Referring to FIG. 2, a downlink sub-frame begins at a preamble used to establish synchronization and equalization at a physical layer. Then, the structure of an overall frame is defined by a broadcast-type downlink map (DL-MAP) message and an uplink map (UL-MAP) message. The DL-MAP message is used to define the location and usage of each burst allocated to the downlink. The UL-MAP message is used to define the location and usage of each burst allocated to the uplink. DL-MAP, DL-MAP IE, UL-MAP, and UL-MAP IE are equal to DL-MAP, DL-MAP IE, UL-MAP, and UL-MAP IE for use in the IEEE 802.16 system capable of supporting the conventional OFDMA system, and have been designed on the basis of the IEEE 802.16e-2005.
The DL-MAP message defines usages allocated to bursts of a downlink interval in a burst-mode physical layer. The UL-MAP message defines usages of bursts allocated to an uplink interval. An information element (IE) constructing the DL-MAP message is used to discriminate downlink traffic intervals of users by a downlink interval usage code (DIUC), a connection identifier (CID), and burst location information (i.e., sub-channel offset, a symbol-offset, the number of sub-channels, and the number of symbols). In the meantime, the information element constructing the UL-MAP message decides its usage by an uplink interval usage code (UIUC) for each CID, and defines the location of a corresponding interval by duration information. In this case, usages of individual intervals are decided according to a UIUC value used in the UL-MAP. Each interval begins at a specific position, which is spaced apart from a previous IE start point by a duration prescribed in the UL-MAP IE.
The UL-MAP message allocates the authority of using an uplink channel. The UL-MAP message defines how to use uplink bursts using consecutive information elements (IEs) capable of defining the method of using individual uplink intervals. The UL-MAP message defines how to use uplink resources allocated as a block unit of either an OFDMA symbol or a sub-channel. The UL-MAP information element (IE) designates band allocation information of an uplink. Each UL-MAP message includes at least one IE to indicate the end of the last burst. The order of IEs transmitted by the UL-MAP message is decided by a used physical layer.
The CID allocates the IE to unicast, multicast, and broadcast addresses. If the allocation of a bandwidth approval is clearly designated, a basic CID of a mobile station (MS) is used as a CID value. The UIUC is used to designate an uplink usage format and associated uplink burst profile. In the case of each UIUC to be used for the UL-MAP, an Uplink_Burst_Profile must be contained in an Uplink Channel Descriptor (UCD). All Information elements (IEs) must be supported by mobile stations. When the UL-MAP message is generated, the base station (BS) is able to freely use any of IEs of the generated UL-MAP message without generating any problems.
Next, a frame structure for supporting the relay station (RS) will hereinafter be described. The RS frame structure establishes a DL relay zone and a UL relay zone of the RS contained in the frame. Data transmitted from the base station (BS) to the relay station (RS) is allocated as a burst format to a frame downlink. Data transmitted from the RS to the mobile station (MS) is allocated to a downlink zone of the relay station (RS). If there is data to be transmitted from the mobile station (MS) to the base station (BS), this data is transmitted from the RS uplink zone to an allocation zone for each MS. The RS includes the resultant data in a zone allocated from an uplink to the RS.
Next, a feedback method of a mobile station (MS) according to a conventional art will hereinafter be described with reference to the frame structure.
In the conventional art of the OFDM or OFDMA wireless communication system, a base station (BS) allocates a predetermined area for receiving a feedback result of a measurement value of a downlink channel status from a mobile station (MS), and requests a downlink QoS (Quality of Signal) from the mobile station (MS). A conventional mobile communication system uses a variety of methods for allowing the base station (BS) to request a downlink QoS from the mobile station (MS), for example, a method of using a Channel Quality Information CHannel (CQICH). In order to allow the base station (BS) to acquire a downlink QoS of the mobile station (MS), the base station (BS) pre-allocates some zones of an uplink two-dimensional (2D) map to a CQICH dedicated channel, and then designates a sub-channel to be used for each MS.
After a CQICH zone is pre-allocated using a fast feedback channel of the UL-MAP IE contained in the UL-MAP message, a slot is allocated to each MS using ‘CQICH_Allocation_IE’ or ‘CQICH_Enhance_Allocation_IE’. The ‘CQICH_Allocation_IE’ message dynamically allocates a CQICH to a MS uplink, or cancels the allocated area. If the CQICH has been allocated to the MS uplink, the mobile station (MS) transmits QoS information to the allocated CQICH area at intervals of a given period prescribed in the ‘CQICH_Allocation_IE’ message. If the CQICH channel to which the mobile station (MS) has been allocated is cancelled, i.e., if a duration parameter of the ‘CQICH_Allocation_IE’ message is set to ‘0’, the mobile station (MS) does not transmit QoS information to the base station (BS). If the relay station (RS) is installed between the base station (BS) and the MS and is then operated, RS-reflected MAC information and associated parameters are adjusted, and the adjusted information and parameters are used. In a broadband wireless access system technology based on the conventional mobile RS, channel quality information (CQI) is transmitted through a fast feedback channel to adjust a Modulation and Coding Scheme (MCS) level and decide a path between the MS and the base station (BS).
The following table 1 illustrates ‘CQICH_Allocation_IE’ prescribed in the conventional IEEE 802.16e-2005.
TABLE 1SyntaxSizeNotesCQICH Alloc IE( ) {Extended UIUC4 bitsCQICH = 0x03Length4 bitsLength of the message in bytes(variable)CQICH_IDVariableIndex to uniquely identify the CQICHresource assigned to the SS. The sizeof this field is dependent on systemparameter defined in UCD.Allocation offset6 bitsIndex to the fast feedback channelregion marked by UIUC = 0.Period (p)2 bitsA CQI feedback is transmitted on theCQICH every 2p frames.Frame offset3 bitsThe SS starts reporting at the frame ofwhich the number has the same 3 LSBas the specified frame offset. If thecurrent frame is specified, the SSshould start reporting in eight frames.Duration (d)3 bitsA CQI feedback is transmitted on theCQI channels indexed by the CQICH_IDfor 10x2d frames. If d==0, the CQICHis deallocated. If d==0b111, the SSshould report until the BS command forthe SS to stop.Report configuration included1 bitUpdate to CINR report configuration isincluded.If (report configuration included ==1){Feedback Type2 bits0b00 = physical CINR feedback0b01 = effective CINR feedback0b10-0b11 = ReservedReport Type1 bit0: Report for preamble1: Report for specific permutation zoneIf (Report type == 0){ CINR preamble report type1 bitThe type of preamble-based CINRreport0 - Frequency reuse factor=1configuration1 - Frequency reuse factor=3configuration}Else {report for permutation zone  Zone permutation3 bitsThe type of zone for which to report0b000—PUSC with ‘use all SC=0’0b001— PUSC with ‘use all SC = 1’0b010— FUSC0b011— Optional FUSC 0b100—SafetyChannel region0b101— AMC zone (only applicable toAAS mode)0b110-111— Reserved  Zone type2 bits0b00— non-STC zone 0b01— STC zone0b10—AAS zone0b11—reserved  Zone PRBS_ID2 bitsThe PRBS_ID of the zone on which toreportIf(Zone type== 0b000 or 0b001)            { Major group indication1 bitIf ‘0’ then the report may refer to anysubchannel in the PUSC zone.If(Major group indication ==1)            {  PUSC Major group bitmap6 bitsReported CINR shall only apply to thesubchannel of PUSC major groups forwhich the corresponding bit is set.Bit #k refers to major group k.}} CINR zone measurement type1 bit0: measurement from pilot subcarrierand, if AAS zone, from AAS preamble.1: measurement from data subcarriers} If(feedback type == 0b00 {Physical CINR feedback Averaging parameter included1 bitIf (Averaging parameter included == 1) { Averaging parameter4 bitsAveraging parameter used for derivingphysical CINR estimates reportedthrough CQICH. This value is given inmultiples of 1/16 in the range of[1/16..16/16] in increasing order. aavg}}}MIMO_permutation_feedback_cycle2 bits0b00=No MIMO and permutation modefeedback0b01=The MIMO and permutation modeindication shall be transmitted on theCQICH indexed by the CQICH_ID everyfour allocated CQICH transmissionopportunity. The first indication is senton the fourth allocated CQICH frametransmission opportunity.0b10=The MIMO mode and permutationmode indication shall be transmited onthe CQICH indexed by the CQICH_IDevery eight allocated CQICHtransmission opportunity. The firstindication is sent on the eight allocatedCQICH transmission opportunity.0b11=The MIMO mode and permutationmode indication shall be transmitted onthe CQICH indexed by the CQICH_IDevery 16 allocated CQICH transmissionopportunity. The first indication is senton the 16th allocated CQICHtransmission opportunity.PaddingVariableNumber of bits required to align to bytelength, shall be set to zero.}
FIG. 3 shows the order of mapping a fast feedback message to the fast feedback area. As can be seen from FIG. 3, some areas of the UL sub-frame are used as the fast feedback channel. The order of channel allocation is equal to that of FIG. 3, and a single sub-channel (i.e., 48 data sub-carriers) is used to represent a single CQI (i.e., 6 bits). Each sub-channel is comprised of six tiles, and 6 tiles are distributed to all bands.
FIG. 4 is a flow chart illustrating a conventional feedback method of a MS channel status using a mobile RS. If a specific base station (BS) communicates with mobile stations (MSs), this base station (BS) is called a serving BS. The serving BS communicates the mobile stations using the mobile RS. In FIG. 4, MMR is indicative of a Mobile Multi hop Relay station. In order to indicate a single hop (i.e., 1 hop) between the serving BS and the RS and a single hop (i.e., 1 hop) between the RS and the MS, abbreviation (MMR) of the Mobile Multi hop Relay station is used.
In more detail, the serving BS allocates feedback resources for three mobile stations (MS#1, MS#2, and MS#3) to the mobile RS during a CQICH_Allocation IE duration using the allocated fast feedback channel at steps S4010, S4020, and S4030. The mobile RS allocates feedback channel resources to individual mobile stations of the above steps S4010˜S4030 at steps S4040˜S4060. Thereafter, individual mobile stations (MSs) feed back channel status information to the RS over a CQICH allocated from the RS at steps S4070˜S4090. The mobile RS receives CQICH feedback information, and transmits the feedback information to the serving BS at steps S4110˜S4120. If the serving BS is in the duration, it returns to the above step S4010 at step S4130.
Due to the use of the RS, a time delay may unavoidably occur in a decoding or encoding process, a system complexity may also unavoidably increase, and there arises a backward compatibility problem of a Point-to-MultiPoint (PMP) mode. If a specific mobile station (MS) uses a mobile RS of the conventional broadband OFDM wireless access system, it should be noted that a method for effectively reporting and managing channel status information is not defined in the above specific MS. Specifically, although a channel status between the mobile RS and the MS is not frequently changed, the base station (BS) must periodically allocate CQICH resources to report channel status information. A single sub-channel of using 48 data sub-carriers is used to represent a CQI indicating single channel quality information composed of 6 bits. If a transmission period of the CQI is short, unnecessary resource consumption occurs due to a frequent CQI channel allocation. If a CQI report period is long, decision of a MS path and a MCS (Modulation and Coding Scheme) level adjustment is delayed.